1. Field of the Invention
This invention relates to refractory materials produced from tailings from asbestos mines. More specifically, this invention yields such refractory materials which are suitable for heat intensive metallurgical applications, such as for example in the taphole nozzles of sliding gate systems in steel mill ladles.
2. Description of the Prior Art
Asbestos deposits normally occur in certain types of silicate rock which contain only about 5 to 10% by volume of asbestos fibres. Consequently, separation of the fibres from asbestos ore leaves large quantities of unwanted tailings which accumulate at or near extraction or processing sites. There is therefore a need to find novel commercial applications for these by-products of asbestos mining and refining operations.
Chrysotile asbestos tailings normally contain a substantial proportion of hydrated magnesium silicates referred to as serpentine. Other components which occur with serpentine rock are brucite Mg(OH).sub.2 and hematite-magnetite (Fe.sub.2 O.sub.3 -Fe.sub.3 O.sub.4). Deposits of nickel may also occur in the rock. Known potential uses for asbestos tailings include the use of the tailings as a source of nickel extracted by magnetic separation of nickel- and iron-rich magnetic components; the transformation of the tailings into mineral wool by melting with carbon and forming fibers by blowing air through the resulting molten silicate; and the use of the tailings as a source of magnesium and/or magnesium compounds obtained by acid leaching and crystallisation.
It is also known to calcine serpentine in order to produce sintered angular shaped granules useful as sandblasting or heat accumulating material, or to produce granular products useful as foundry mold sands. However, conventional calcination methods give rise to a number of drawbacks. For example, the conventional process of calcining serpentine results in the production of a synthetic harzburgite rich in a pyroxene of the silicate group and commonly referred to as enstatite. Enstatite is further subdivided into protoenstatite and clinoenstatite depending on its resulting crystallographic structure. However, these products have relatively low melting or softening points (less than 1,600.degree. C.), and are therefore not useful as refractory materials for particularly high temperature operations, e.g. in the production of steel which involves exposure to temperature in the region of 1,700.degree. C. for several hours.
The known methods of serpentine calcination normally involve heating the serpentine to a temperature of about 1,300.degree. C., whereby the following chemical reactions are said to take place:
(i) dehydration encountered at 600.degree. to 780.degree. C. to form an anhydrous magnesium silicate: ##STR1## (ii) conversion of the anhydrous magnesium silicate into forsterite (Mg.sub.2 SiO.sub.4) and free silica (SiO.sub.2), which starts to occur at approximately 800.degree. to 900.degree. C.: ##STR2## (iii) reaction of forsterite with free silica above 1,000.degree. C. thereby forming enstatite (MgSiO.sub.3): ##STR3##
As previously stated, such enstatite-rich products can find application as, for example, foundry mold sands or backing sand mixes. However, the product is unsuitable for higher temperature applications in which the product is required to have superior refractory properties, for example, in steel or other high temperature smelting operations. More specifically, enstatite rich-products are unsuitable as ladle and ladle nozzle sands currently utilized, for example, in steel mills and foundries because the product is unable to withstand the necessary temperatures and holding times involved.
Sands currently used in steel mill ladles are mostly silica, zircon and chromite based. Although such products have good granulometry, packing density and purity, they are expensive to produce. Moreover, many of these products have various degrees of toxicity. Indeed, the manipulation and use of many of such products causes the liberation of silicious dusts which are known to engender among workers the serious medical condition known as silicosis.
To produce a synthetic olivine from asbestos tailings suitable for extremely high temperature applications, it is theorized that a high proportion of forsterite (Mg.sub.2 SiO.sub.4) should be produced in the final product. This is because of the favorable refractory properties of forsterite compared to enstatite and because the fusion point of forsterite is much higher than that of enstatite. Aitcin, in Am. Ceramic Society Bull., Vol. 61, No.8 (1982) pp. 857 to 860, teaches calcining and sintering non-magnetic portions of asbestos tailings to produce a refractory product. A process is described whereby fine asbestos tailings are fired at 1,550.degree. C. However, the resulting refractory product is rich in enstatite. The forsterite content of the refractory product will be a function of the basicity index (MgO:SiO.sub.2) of the asbestos tailings. Hence the forsterite content of the final product is limited by the basicity index of the raw material. For example, a typical fusion product of calcined asbestos tailings consists of 33% by volume of forsterite and 54% by volume of enstatite, the remainder being essentially hematite (Fe.sub.2 O.sub.3).
Accordingly, there remains a need to realize an efficient method of producing a synthetic olivine composition from asbestos tailings for applications such as, for example, high temperature ladle nozzle sands, which method will overcome the drawbacks of the prior art.
A first object of the present invention is to provide a process of treating asbestos tailings in order to convert such tailings into a commercially useful product.
A second object of the invention is to provide a process of producing, from asbestos tailings or other ores of similar composition, synthetic olivine sands having good refractory characteristics at very high temperatures, such as those encountered during the manufacture and treatment of steel in steel mill foundries.
Another object of the invention is to produce a forsterite-rich product from a serpentine mineral or from a previously-calcined serpentine mineral containing enstatite.
Another object of the invention is to provide a novel refractory material suitable for use at very high temperatures.
Yet another object of this invention is to provide a novel non-toxic sand exhibiting higher refractoriness, greater density and a lower coefficient of thermal expansion than many conventional high temperature foundry sands.